The present invention relates to a process for obtaining light olefins by the dehydrogenation of the corresponding paraffins, in particular C2-C20 (paraffins with 2 to 20 carbon atoms).
Olefins are important intermediates for the production of chemicals having a wide distribution such as: polypropylene, antiknocking additives (MTBE), fuels with a high octane number, alkylated derivatives and numerous other products.
In spite of the growing demand for these derivatives, the expansion of industrial processes for their preparation is often limited by the restricted availability of olefins, for example isobutene in the production of MTBE.
This has led to identifying other sources of olefin supply, together with the traditional ones (FCC, Cracker). Among these the source which is becoming more and more important is represented by the dehydrogenation reaction of light paraffins. This, although simple from a stoichiometric point of view, has problems with respect to thermodynamics and kinetics. The reaction is endothermal and is regulated by thermodynamic equilibrium; this leads to the necessity for temperatures higher than 500xc2x0 C. for dehydrogenating C2-C4 paraffins with economically acceptable conversions per passage. In addition it is necessary to supply the system with heat because of the endothermal nature of the reaction.
In spite of the high operating temperatures the dehydrogenation rate is low and it is consequently necessary to operate in the presence of a suitable catalyst. The latter must be thermally stable and capable of guaranteeing high selectivities towards the desired olefin, minimizing isomerization, cracking, coking and aromatization side-reactions and ensuring industrially useful conversion values.
The inevitable formation of coke on the catalyst causes a progressive reduction in the catalytic activity and it is therefore indispensable to carry out periodic regenerations.
As a result the formulate must have a high stability under the conditions to which it is subjected during the reaction and regeneration phases.
Several efforts have been made to identify catalytic compositions which can satisfy the demands imposed by the type of process.
Patent literature in fact, cites several catalytic compositions based on noble metals and combined with other chemical species (U.S. Pat. No. 3,531,543, U.S. Pat. No. 4,786,625; U.S. Pat. No. 4,886,928; EP-351067) and also based on metal oxides in the presence of promoters, in most cases consisting of supported Cr2O3 (U.S. Pat. NO. 2,945,823; U.S. Pat. No. 2,956,030; U.S. Pat. No. 2,991,255; GB-2162082).
Both groups of formulations, however, have disadvantages: those based on noble metals require particular treatment in the regeneration phase (U.S. Pat. No. 4,438,288) to preserve the dehydrogenating activity of the metallic species, resorting for example to post-treatment with chlorinated substances and subsequent reducing treatment; those based on chromium oxide, supported on alumina, silica, silica-alumina, etc., are characterized in that they have a low selectivity to olefin owing to their acid nature which causes parasite reactions such as isomerization, cracking, coking and aromatization which are typical acid catalyzed reactions.
The selectivity to olefin is increased by modifying the formulations with the addition of alkaline and/or earth-alkaline metal oxides to mitigate the acid properties.
Literature discloses (J. Phys. Chem., Vol. 66, 1962) that the charging high quantities of alkaline oxides, with the aim of improving the selectivity, jeopardizes the catalytic performance of the formulates: the strong interactions with the chromium oxide suppress the dehydrogenating activity, whereas the residual chromium with an oxidation state of more than ÷3, which cannot be completely reduced as it is stabilized by the high alkyline charging, decreases the selectivity to the desired olefin.
We have surprisingly found that by using a particular catalytic system mainly consisting of Cr2O3, supported on an alumina modified with silica, to which tin oxide is added, the selectivity to the desired olefin is significantly improved.
The addition of tin drastically reduces the formation of products deriving from acid catalyzed side-reactions with a beneficial effect on the selectivity to olefin.
The process for obtaining light olefins by the dehydrogenation of the corresponding paraffins, object of the present invention, consists:
a) in reacting in a reactor, operating at a temperature of between 450 and 800xc2x0 C., at a pressure of between 0.1 and 3 Atm absolute and with a GHSV space velocity of between 100 and 10000 hxe2x88x921, said paraffins with a catalytic system containing chromium oxide, tin oxide, at least one alkaline metal oxide (M) and an alumina carrier, in delta or theta phase or in mixed delta+theta or theta+alpha or delta+theta+alpha phases, modified with silica, in which:
the chromium, expressed as Cr2O3, is in a quantity of between 6 and 30% by weight, preferably between 13 and 25%;
the tin, expressed as SnO, is in a quantity of between 0.1 and 3.5% by weight, preferably between 0.2 and 2.8%;
the alkaline metal, expressed as M2O, is in a quantity of between 0.4 and 3% by weight, preferably between 0.5 and 2.5;
the silica is in a quantity of between 0.08 and 3% by weight,
the complement to 100 being alumina,
in regenerating said catalytic system in a regenerator by burning the coke deposited on its surface operating at a temperature of more than 400xc2x0 C.